refactor(shm/kv) retrieve items by key hash

The change is motivated by the upcoming proxy-wasm metrics support, which will
be built atop `ngx_wasm_shm_kv`.

Proxy-Wasm filter developers are expected to define metrics before using them.
When a metric is defined the host returns an id to the filter which is later
used for updating or retrieving that metric's value.

As such, now `ngx_wasm_shm_kv_get_locked` expects either a key (ngx_str_t *)
or its hash (uint32_t *). If both are provided, the hash is used and the key
is ignored.

A new function `ngx_wasm_shm_rbtree_lookup` is also introduced allowing
item retrieval using only key hash.

src/common/proxy_wasm/ngx_proxy_wasm_host.c

@@ -1283,7 +1283,7 @@ ngx_proxy_wasm_hfuncs_get_shared_data(ngx_wavm_instance_t *instance,
 
     ngx_wasm_shm_lock(resolved.shm);
 
-    rc = ngx_wasm_shm_kv_get_locked(resolved.shm, &key, &value, cas);
+    rc = ngx_wasm_shm_kv_get_locked(resolved.shm, &key, NULL, &value, cas);
 
     ngx_wasm_shm_unlock(resolved.shm);
 

src/common/shm/ngx_wasm_shm_kv.c

@@ -133,14 +133,45 @@ queue_for_node(ngx_wasm_shm_t *shm, ngx_wasm_shm_kv_node_t *n)
 }
 
 
+static ngx_wasm_shm_kv_node_t *
+ngx_wasm_shm_rbtree_lookup(ngx_rbtree_t *rbtree, uint32_t key_hash)
+{
+    ngx_wasm_shm_kv_node_t  *n;
+    ngx_rbtree_node_t       *node, *sentinel;
+
+    node = rbtree->root;
+    sentinel = rbtree->sentinel;
+
+    while (node != sentinel) {
+
+        n = (ngx_wasm_shm_kv_node_t *) node;
+
+        if (key_hash != node->key) {
+            node = (key_hash < node->key) ? node->left : node->right;
+            continue;
+        }
+
+        return n;
+    }
+
+    return NULL;
+}
+
+
 ngx_int_t
 ngx_wasm_shm_kv_get_locked(ngx_wasm_shm_t *shm, ngx_str_t *key,
-    ngx_str_t **value_out, uint32_t *cas)
+    uint32_t *key_hash, ngx_str_t **value_out, uint32_t *cas)
 {
-    ngx_wasm_shm_kv_node_t  *n;
     ngx_wasm_shm_kv_t       *kv = ngx_wasm_shm_get_kv(shm);
+    ngx_wasm_shm_kv_node_t  *n;
 
-    n = (ngx_wasm_shm_kv_node_t *) ngx_str_rbtree_lookup(&kv->rbtree, key, 0);
+    if (key_hash) {
+        n = ngx_wasm_shm_rbtree_lookup(&kv->rbtree, *key_hash);
+
+    } else {
+        n = ngx_wasm_shm_rbtree_lookup(&kv->rbtree,
+                                       ngx_crc32_long(key->data, key->len));
+    }
 
     if (n == NULL) {
         return NGX_DECLINED;
@@ -251,11 +282,13 @@ ngx_wasm_shm_kv_set_locked(ngx_wasm_shm_t *shm, ngx_str_t *key,
     ngx_str_t *value, uint32_t cas, ngx_int_t *written)
 {
     size_t                   size;
-    ngx_wasm_shm_kv_node_t  *n, *old;
+    uint32_t                 key_hash = ngx_crc32_long(key->data, key->len);
     ngx_wasm_shm_kv_t       *kv = ngx_wasm_shm_get_kv(shm);
+    ngx_wasm_shm_kv_node_t  *n, *old;
 
     old = NULL;
-    n = (ngx_wasm_shm_kv_node_t *) ngx_str_rbtree_lookup(&kv->rbtree, key, 0);
+    n = (ngx_wasm_shm_kv_node_t *) ngx_wasm_shm_rbtree_lookup(&kv->rbtree,
+                                                              key_hash);
 
     if (cas != (n == NULL ? 0 : n->cas)) {
         *written = 0;
@@ -318,6 +351,7 @@ ngx_wasm_shm_kv_set_locked(ngx_wasm_shm_t *shm, ngx_str_t *key,
 
         n->key.str.data = (u_char *) n + sizeof(ngx_wasm_shm_kv_node_t);
         n->key.str.len = key->len;
+        n->key.node.key = ngx_crc32_long(key->data, key->len);
         n->value.data = n->key.str.data + key->len;
         n->value.len = value->len;
 

src/common/shm/ngx_wasm_shm_kv.h

@@ -15,7 +15,7 @@ typedef struct {
 
 ngx_int_t ngx_wasm_shm_kv_init(ngx_wasm_shm_t *shm);
 ngx_int_t ngx_wasm_shm_kv_get_locked(ngx_wasm_shm_t *shm,
-    ngx_str_t *key, ngx_str_t **value_out, uint32_t *cas);
+    ngx_str_t *key, uint32_t *key_hash, ngx_str_t **value_out, uint32_t *cas);
 ngx_int_t ngx_wasm_shm_kv_set_locked(ngx_wasm_shm_t *shm,
     ngx_str_t *key, ngx_str_t *value, uint32_t cas, ngx_int_t *written);
 ngx_int_t ngx_wasm_shm_kv_resolve_key(ngx_str_t *key,